The Big Question: What does the Territorial Army do, and how has its role changed?

Why are we asking this now?

Because in the year of its centenary, the Territorial Army, made up of people from all walks of life who give up weekends, holidays and even head off on six-month stints on active duty abroad, has been hit by tragedy this week. Three reservists, Corporal Sean Robert Reeve of the Royal Signals, Lance Corporal Richard Larkin and Paul Stout, were killed alongside Corporal Sarah Bryant of the Intelligence Corps when their armoured Land Rover was hit by a roadside bomb. The nature of their mission has shattered old impressions of exactly what TA regiments do. Far from guarding bases or making the grub, the three men from the 23rd SAS regiment were part of a secret counter-intelligence mission, monitoring Taliban activity.

The incident, which was the biggest loss of life for the TA since the Second World War, underlined the fact that TA soldiers are now a key plank of Britain's defence forces. The old tags of "Sunday soldiers" or "weekend warriors" seem well and truly out of date.

What kinds of people join the TA?

Just like those ever-present recruitment TV ads say, recruits are plumbers, builders, lawyers and even MPs – David Davis served his time in the TAs, and Winston Churchill was the commanding officer of a TA regiment, the Oxfordshire Hussars. Comedians have also mucked in. Billy Connolly took to the skies with the 15 Parachute Regiment, completing several jumps, though he never saw active service. But its most notable alumnus has to be one 2nd Lieutenant Elizabeth Windsor. The then Princess swapped her crown for a crankshaft during the Second World War as a mechanic with the Auxiliary Territorial Service.

There is a minimum period of service in the TA of three years. Recruits are free to leave after that if they want, though they can extend their service for several years at a time. Most will have to train for between 19 to 27 days each year, including a two-week annual camp, so TA soldiers have to sacrifice a good slice of their free time.

Numbers have fallen dramatically since 1997, with some blaming the lengthy tours of duty many are asked to perform on account of the UK's major overseas military commitments.

Can TA troops really be in the SAS?

It may come as a surprise to many that even the SAS, known to the world as the British armed force's crack team of elite soldiers, has a TA regiment attached to it. In fact, it has two – the Artists Rifles and the 23rd SAS regiment, in which the soldiers involved in the latest incident served. TA soldiers serving in the regiments are very highly regarded, with some placing their skills higher than many regular soldiers. The selection process is tough, and not far off that subjected on full-time SAS hopefuls. Men aged between 18 and 30 are free to apply (the age limit is extended for those with military experience). Then a series of mental and physical tests are used to highlight the cream of the crop. Training takes place just like other TA soldiers, over a series of weekends and camps.

What kind of tasks are they given?

It can be hard to tell at any one time, because their missions are carried out in secret. Information on their whereabouts emerges only sporadically. The SAS reservists killed this week were part of an operation run by the Afghan National Police near Lashkar Gar, in Helmand Province. The mission involved listening in on Taliban communications. The work demonstrated the key role reservists have come to play in Afghanistan and Iraq, where SAS reservists have also been deployed.

The SAS reserve regiments are far from the only TA units put in harm's way, though. Others have demonstrated bravery as great as any full-time soldier. Earlier this month, Luke Cole, attached to the 2nd Battalion Mercian Regiment, was handed the Military Cross for the courage he demonstrated in Afghanistan while on tour last year. And around 1,200 TA troops still support regular troops each year in Iraq, Afghanistan and the Balkans. Numbers reached a peak at the height of the Iraq invasion, in March 2003, when 46,000 British troops were involved.

Has the TA changed much over the years?

Certainly from the loose coalition of mounted yeomanry units made up of farmers and tenants that made up volunteer armies before the TA was founded on 1 April, 1908. But ever since the First World War, when the TA was first mobilised, serving TA soldiers have quickly become almost indistinguishable from their full-time comrades. But while the commitment of its troops may have stayed the same, the TA's size has fluctuated. After doubling in size during the Second World War, its membership and resources dwindled thereafter. By the early 1990s, its reputation had similarly shrunk, seen as good for little more than guarding the home front.

How about now?

Operations in Afghanistan and Iraq stretched the British armed forces to their very limits. Suddenly the TA found itself catapulted into a far more important role. It switched from becoming an under-used reserve force to in-demand battalions seeing active service.

Nearly 7,000 TA soldiers found themselves on their away to Iraq in 2003 as part of Operation Telic, the name given to British participation in the country. With the increased activity came a change in rhetoric from Whitehall. Another restructuring of the Army in 2004 saw the government change the TA's status to reserves of the regular regiments – an acknowledgement that its recruits were now being asked to do much more. The 15 TA infantry battalions were squashed down into 14, though numbers remained the same.

Are they derided by full-time troops now?

Not for long, according to Kevin Mervin, a former TA soldier who served as a recovery mechanic during the Iraq invasion. He says that any mutual suspicions soon wear off. "There is a bit of animosity at first, but once you prove that you can do the job, you soon become one of the regiment," he said. "To all intents and purposes, you're then a regular soldier. Your training is the same – you are part of that unit. And because you have life experience, some of the younger guys also look up to you for guidance. And you don't join any of the armed services without wanting to one day prove you can carry out your training for real." His case also shows the commitment of TA soldiers – he lost a new job due to his tour of duty in Iraq.

He's not the only one to have won over the regulars. As he lay dying on the muddy fields of the Somme in 1916, Brigadier-General Charles Prowse declared himself a convert. "I did not think much of Territorials before," he said. "But by God they can fight."

Should TA troops be regarded as more than part-time soldiers?

Yes...

* No one joins any branch of the armed services without wanting to put their training into action

* The commitment and skill of TA recruits is shown by the numerous medals and praise showered upon them by senior army figures

* Once a TA soldier is attached to a regiment, they are expected to behave as a full-time soldier

No...

* They may train hard, but their weekend and occasional camp training cannot match that of a full-time soldier

* Numbers in the TA have fallen, perhaps put off by the increased likelihood of seeing action

* As soon as Britain's full-time armed forces are less stretched, the TA will be regarded as less important again.